Method for selecting modulation tiers for transmissions over an orthogonal frequency division multiplexed (OFDM) channel
With the introduction of orthogonal frequency division multiplexing (OFDM) technology to communication channels in a cable network, cable modems that receive data downstream over subcarriers of the OFDM channel are grouped into modulation tiers based on at least one metric. Modulation tiers are generated at a subcarrier group level such that network elements can use different profile tiers depending in the subcarrier group in use. A scheduler facilitates the scheduling of modulation tiers for network element destination groups across subcarrier groups, where the modulation tier for a network element may vary between subcarrier groups. The scheduler also facilitates scheduling multiple network element destination groups for transmission of a plurality of subcarrier groups for optimal transmission to the network elements.
1. A method for selecting a modulation tier for transmitting data elements over an orthogonal frequency division multiplexed (OFDM) channel to a network element destination group, the method comprising:
identifying a first subcarrier group in an OFDM channel available for scheduling a transmission of the data elements, wherein the OFDM channel is comprised of a plurality of subcarrier groups for scheduling transmissions to a plurality of network element destination groups, each subcarrier group assigned to serve one or more network elements within a network element destination group;
identifying a network element destination group including one or more network elements identified for receiving the data elements;
determining the modulation tier projected for transmitting data elements during the first subcarrier group to the network element destination group, the projected modulation tier determined as a function of network elements associated with the network element destination group wherein projecting the modulation tier comprises:
identifying one or more modulation tiers mapped to at least one network element in the network element destination group during the subject subcarrier group; and
projecting the modulation tier for transmitting at least one data element to the network element destination group as a function of the available subcarrier group and the one or more modulation tiers mapped to the network element destination group;
comparing the modulation tier projected for the network element destination group during the first available subcarrier group to at least one of:
at least one modulation tier projected for the network element destination group if served during one or more of the other available subcarrier groups in a same transmission over the OFDM channel, or
at least one modulation tier projected for the network element destination group if served by a subcarrier group on a later transmission over the OFDM channel;
selecting between scheduling the transmission of the data elements to the network element destination group during the first subcarrier group or to reschedule transmission of the data elements to the network element destination group to a different subcarrier group in the same or a different OFDM channel, wherein determining whether to schedule the transmission to the network element destination group based on at least two of:
whether a higher order modulation option is available during another subcarrier group in the OFDM channel for the network element destination group;
whether another network element destination group has data awaiting transmission; or
whether reorganizing the schedule of transmissions optimizes the performance of the network elements in more than one network element destination group; and
sending the data elements modulated according to the modulation tier associated with the selected subcarrier group for the network element destination group.
2. The method of claim 1 , wherein the network element destination group comprises at least one network element.
3. The method of claim 1 , wherein the transmission to the network element destination group is scheduled to a different subcarrier group due to a higher order modulation projected for the network element destination group during the different subcarrier group.
4. The method of claim 1 , wherein the network element destination group is a first network element destination group and the transmission to the first network element destination group during the available subcarrier group is not scheduled, and a transmission to a second network element destination group during the available subcarrier group is scheduled.
5. The method of claim 4 , further comprising:
selecting a modulation tier for the second network element destination group during the available subcarrier group; and
sending data modulated according to a selected modulation tier for the second network element destination group.
6. The method of claim 1 , further comprising identifying one or more modulation tiers mapped to network elements across subcarrier groups in the OFDM channel.
7. The method of claim 1 , wherein the one or more modulation tiers are mapped to the network elements based on measurements collected for the network elements and at least one metric defined for each of the plurality of tiers.